Skip to main content

Full text of "Text Book Of Mechanical Engineering"

See other formats

Appendix L                                773

P. 650. An Isochronous Governor is a governor having
but one speed consistent with stability. A parabolic governor is
isochronous excepting for the influence of friction, its stable speed
occurring when the balls are in their lowest position. If this
speed be increased the equilibrium is neutral, because the height
of the cone does not change with the rise of the balls. All over-
sensitive governors hunt more or less, that is, they are apt to rise
too high and fall too low, even with the small changes of load
during a revolution, and the result is a condition of oscillation
which prevents a settled position corresponding to engine speed.
This extreme sensitiveness can be prevented by a spring (Fig.
645), but in a much better way by a dashpot (Fig. 261), for in
the latter case the air causes a constant though small resistance.

P. 700. Ignition Tubes.—Porcelain tubes have been used
for some time (1895), their life being about twelve months.
Iridio-platinum is also now adopted (1898) with great success.

Description of an Engine for Refrigerating Air.—
When work is done on a gas, without subtraction of heat, the
whole work is expended in raising the temperature of the gas;
and when a gas does work without addition of heat the whole
work is obtained by the abstraction of heat from the gas, causing
a decrease of temperature. In practice these results are only
partially effected, but the general changes obtained may be
understood by reference to Fig. 562, and P. 547, where
(theoretically) air at 60° F. is cooled to - 201° F.

The work derived from the expansion of steam in the engine
cylinder is employed to drive the piston of an air-compressing
cylinder, which draws air on the inner and compresses it on. the
outer stroke, up to 90 or loolbs by gauge. The temperature
rising to 200° or 300°, the cylinder is jacketed with cold circu-
lating water, to prevent damage to lubricants, and thus the air is
partially, cooled. Leaving this cylinder, it passes through the
pipes of a surface condenser, being there cooled to 60° or 70° by
cold water circulation round the tubes; and from the condenser
it enters an expansion cylinder covered with a non-conductor.
Here the air does work, helping the engine, and, cooling to